A variety of attempts have been made to improve external quantum efficiency of a light-emitting device. Among them, methods for roughening an exposed surface of a light-emitting device have been presented.
U.S. Pat. No. 6,504,180 related to a GaAs based light-emitting device discloses a technology wherein at least some of an exposed surface of a light-emitting device is roughened so as to improve external quantum efficiency.
The reason why a desired portion of the exposed surface of the GaAs based light-emitting device can be made rough freely is that GaAs has the material property in which it can be easily machined.
However, a GaN based light-emitting device has lots of limitations in machining an exposed surface unlike a GaAs based light-emitting device. These limitations may include that it is difficult to machine an n-type layer since a substrate such as sapphire is provided in the n-type layer, growth of a thick p-type GaN layer results in an increase in crystalline defects even though the thick p-type GaN layer is required for machining, etc. It is therefore difficult to apply such a technology applied to the GaAs based light-emitting device to the GaN based light-emitting device without hard work. In order to improve external quantum efficiency by roughening the exposed surface of the GaN based light-emitting device, it will need an approach based on understanding of the GaN based light-emitting device itself. Furthermore, according to U.S. Pat. No. 6,504,180, polystyrene spheres are used as a mask in order to make an exposed surface rough. It is, however, difficult to apply an etching technology, which uses polystyrene spheres as a mask, to the GaN based light-emitting device.
U.S. Pat. No. 6,441,403 regarding the GaN based light-emitting device discloses a light-emitting device in which a roughened surface is formed on a p-type Al(x)Ga(y)ln(1-x-y)N layer epitaxially grown on an active layer or an n-type Al(x)Ga(y)ln(1-x-y)N layer epitaxially grown on an active layer. Such a structure shows that it is difficult to form a roughened surface on an n-type Al(x)Ga(y)ln(s-x-y)N layer in a conventional light-emitting device structure in which the n-type Al(x)Ga(y)ln(1-x-y)N layer is located below an active layer.